Local Atomic and Electronic Structure

Objectives

• Establish the adsorption sites and geometries of bare linker units and porphyrins with added linker units on magnesium and cobalt oxides (thin film or bulk), the influence of defects, oxide film thickness, and surface termination.
• Investigate the electronic structure of individual porphyrins on MgO and cobalt oxide surfaces: frontier orbitals, local work function and charge distribution.
• Analyze the lateral homogeneity and quality of self-assembled molecular layers.
• Quantify the energy barriers for diffusing molecules on MgO and cobalt oxide surfaces.

The atomic scale insight provided in this project will clarify important mechanisms on the molecular level to achieve the funCOS vision of functional landscaping of oxide surfaces.

Systems and strategy

funCOS 1 employs STM and AFM in UHV. Oxide surfaces (MgO, CoO) will be prepared either by cleaving single crystals and suitable post-processing in UHV or by growing well-defined oxide layers on clean metal surfaces in UHV (e.g., MgO on Ag(100) and CoO on Ir(100)). Defects on the oxide surfaces will be deliberately introduced by ion bombardment. After characterization of the oxide surfaces and their characteristic defects by AFM, STM, and also LEED, molecules will be deposited from the gas phase in UHV. If necessary, methods and parameters for bringing the various molecules into the gas phase under UHV conditions will be developed in cooperation with our funCOS partners. Simple methods like thermal evaporation of molecular powders or dosing molecules from a liquid supply have been tested and used in the two workgroups of funCOS 1. With these methods the coverage of molecules can be chosen from sub-monolayer to the monolayer regime for studying the properties of individual adsorbed molecules but also of self-assembly and inter-molecular interaction. Molecular adsorption can take place at temperatures ranging from 40 to 1000 K. Additionally various metals can be co-adsorbed.

The molecules used in funCOS 1 come from the funCOS Molecular Toolbox. At the beginning of the project various funCOS Test Molecules will be deposited on the oxide surfaces and their adsorption properties studied. Since the molecules can only interact either via the functional group or the benzene π-system the relevant interaction can be properly determined. In view of the funCOS vision of functionalizing oxide surfaces by organic molecules the investigation of the interaction of porphyrins will consume the major share of the project duration. The large molecules will allow a detailed mapping of electronic properties.

As a start, we will consider singly functionalized molecules but for steering the self-assembly of these, multiply functionalized molecules will also be considered and chosen carefully after consultation with our funCOS partners. Self-assembly processes will also be modified by adding metal atoms to the molecular overlayer in UHV. This can either be accomplished in simultaneous co-adsorption of molecules and metal atoms or by seeding oxide surfaces with metal atoms prior to molecular adsorption or by evaporation of metals onto an existing oxide–molecule system. The setups will also allow co-adsorption of more than one molecular species, however, such experiments are not foreseen during the currently requested funding period.

The molecular systems prepared will be transferred in situ to the scanning probe instruments primarily working at liquid helium temperatures for stability and sensitivity reasons. However, working at other fix-point temperatures (liquid nitrogen and room temperature) and within temperature ranges of ~10 K around these fix-points is also possible. funCOS 1 workpackages WP 1 and WP 2 require atomic- or molecular-scale lateral resolution of the molecule–oxide system, WP 3 makes use of spectroscopic methods. The planned experiments require a extreme stability of the instruments but also of the adsorbate on the substrate surface that can only be guaranteed by a low-temperature environment.

Not only the starting point of the project (funCOS Showcase systems) but also the subsequent experiments will be conducted in close collaboration with other funCOS groups to maximize the synergetic effect inherent in the complementarity of funCOS methods. funCOS 1 will provide local site information to the other funCOS projects and will help to unravel molecular configurations encountered in adsorption or self-assembly. Further, ensemble averaging structural and spectroscopic experiments carried out by the funCOS partners and the support from theory in funCOS 6 will provide a solid ground on which the scanning probe experiments can be interpreted.